Various flow meters are available for use in measuring the flow rate of a fluid flowing through a fluid line. The operation of different types of flow meter are based on different techniques of flow measurement. An example of one type of flow meter is the displacement flow meter. The displacement flow meter measures fluid flow by measuring the displacement of a movable boundary, such as a piston or a diaphragm, which moves in response to the fluid flow. The movable boundary in part defines a chamber into which the fluid flows. By closing a valve in the fluid flow path with which the chamber communicates, flow through the chamber is temporarily confined such that the fluid accumulates in the chamber, resulting in an increase in the volume of the fluid in the chamber. As its volume increases, the fluid moves (displaces) the movable boundary. The increase in fluid volume in the chamber, and the attendant displacement of the movable boundary, can be correlated to the flow rate of the fluid. The displacement flow meter includes a transducer or sensor configured to measure the displacement of the movable boundary and generate an electrical signal representative of the amount or rate of displacement. The flow measurement signal is processed by appropriate electronics to calculate the fluid flow rate.
The displacement flow meter is advantageous in that its ability to measure fluid flow, and the accuracy of such measurement, are largely insensitive to the physical parameters of the fluid being measured such as fluid composition, viscosity, temperature, density, compressibility, etc. One example for which flow measurement independent of or unaffected by fluid parameters is advantageous is fluid flow through a fluid line of a chromatography instrument. The fluid flowing through a chromatographic conduit is typically a mixture of different components, the composition of which can vary significantly over time. Within the class of displacement flow meters are those utilizing an acoustic displacement transducer (ADT) as the flow sensor. The ADT utilizes a compliant diaphragm as the movable boundary, which may be similar to the diaphragm of an audio loudspeaker. An electromagnetic coil is mounted so as to move with the diaphragm. Movement of the coil in a static magnetic field induces a current in the coil that is utilized as the flow measurement signal. Examples of ADT-based flow meters are described in U.S. Pat. Nos. 5,460,038 and 5,540,104, the contents of which are incorporated herein by reference.
As with many types of measurement instruments, the calibration of a flow meter drifts over time with continued use of the flow meter. Thus, a flow meter needs to be recalibrated periodically. Recalibration of a handheld flow meter typically requires that the user return the entire flow meter to the manufacturer or other service provider having the expertise needed for performing the recalibration. The act of returning the flow meter is disruptive to the user, as the flow meter is unavailable for use during this time. For a remote user such as a user located in a different country, the downtime required to ship the flow meter to the manufacturer, perform the calibration, and ship the flow meter back to the user can span several weeks and the shipping costs can be high. Therefore, it would be desirable to provide a flow meter that enables replacement of components, calibration or other modification to be performed in an easier and more efficient manner, and which preferably eliminates the need to return the flow meter to the manufacturer or other entity remote from the user.
Certain components of the flow meter, such as sensitive electronic components, may be adversely affected by exposure to the fluid being measured by the flow meter. Moreover, certain types of fluid that may be measured may react to heat or electromagnetic energy emitted by electronic components in an adverse way. Therefore, it would also be desirable to provide a flow meter that directs fluid flow away from certain electronic components and/or other components of the flow meter, or otherwise isolate such components from the fluid.